Large battery systems, which are based for example on lithium technology, for example lithium ions, lithium polymer, lithium sulfur, are used in various applications. Usually, a battery system comprises 50 to 200 cells. In large energy stores, several thousand cells is also usual.
Typically, 6 to 12 cells are used for each module, the number of cells generally being limited by a module voltage of 50 V. Above 50 V, the modules are deemed to be high-voltage components.
The cells may be present in various configurations, specifically as round cells having a solid metal housing, as prismatic cells having a solid metal housing, or as pouch cells having a flexible foil housing.
Pouch cells have major advantages over cell forms having solid housings.
The energy density or power density is higher, because the housing weighs less.
The manufacturing costs are much lower, since the housing is cheaper.
The assembly of an electrode/separator stack in the housing can be integrated into a lamination manufacturing process.
The cell size can be changed in a simple manner, since larger or thicker cells do not require a new cell housing. Cell manufacturers can thus react rapidly to consumer requirements.
Pouch cells promote future trends in active material developments. For example, anode materials based on a non-graphite substrate have a much higher volume work in cyclization, leading to a higher expansion in thickness of the cell housing. This swelling can only be tolerated to a limited extent by solid housings. Further, the pouch cell design is also applicable to future cell chemistry designs, for example alkali metal sulfur designs.
However, the following technical drawbacks present an obstacle. Mounting the flexible cells in a module housing or system housing is complex. The cell thickness changes during charging. In the event of a fault, the location of the cell opening is not predictable.
The following table compares the types of cells.
Round cellPrismatic cellPouch cellSystem:wound electrodesstackedstacked or woundand separatorelectrodeselectrodes andmetal housingand separatorseparatormetal housingfoil housingTechnicalCooling isNo dead volumeNo dead volumeadvantageseffectivebetween cellsbetween cellsFixed electrodesFixed electrodesScalabilityStable housingStable housingCooling in theMechanicallyMechanicallycell isrobustrobusteffectiveCooling in theLightweightcell isLower productioneffectivecostsTechnicalHeavyHeavyMounting inchallengesCooling in theRapid gashousingcell isdischarge in theCell thicknessineffectiveevent of a faultchanges duringLimitedLimitedchargingscalabilityscalabilityLocation of cellDead volumeopening notbetween cellspredictable inthe event ofa fault
Pouch cells for the aforementioned applications have the following typical dimensions. The surface area is approximately 100×100 mm2 to 300×400 mm2. The thickness is approximately 8-15 mm. An increased cell size may for example bring about an increased cell capacity, for example for energy batteries, or else an increased cell power (in power batteries). Changing the active material may also necessitate adaptation of the cell dimensions. For the same system parameters and when using an active material having a lower energy density, thicker cells may be required.
Fixing and/or mounting pouch cells in a housing is already disclosed in the prior art. Substantial drawbacks of the previous fixing designs are as follows. The cells are embedded in a fairly complex frame. A new cell size therefore requires a completely new frame. This in turn generally involves high investments, for example for new tools if plastics frames or polymer frames are used. This means it is not possible to effectively take advantage of one of the major advantages; the possibility of rapid and cost-effective scalability of the cell size.
Further, the manufacturing processes for frames are slow by comparison with those for cells. Accordingly, either the frame production process is the bottleneck of battery system production, or the frames have to be made in a plurality of parallel systems or tools. This again requires high investments.